In the tertiary hydrocarbon-containing class of silanes, the thexyl- and tert-butyl-substituted silanes are certainly the most investigated and most used silanes of this class. Thexyl-substituted silanes are in principle accessible via hydrosilylation reactions between an Si-H compound and 2,3-dimethyl-2-butene. Such a synthesis is described in EP-A-177 454. However, undesirable by-products, some of which are difficult to remove, occur in these reactions due to migration of the double bond.
A. Shirata, TETRAHEDRON LETT. 30 (1989) p. 6393, describes the preparation of tert-butyl-substituted silanes by reaction of halogenosilanes with tert-butylmagnesium chloride, catalysts being necessary to effect the reaction.
U.S. Pat. No. 5,332,853 describes the synthesis of tert-butylsilanes starting from tert-butyllithium. Although this reaction proceeds with quite good yields, it is of little economic interest because of the high price of the lithium alkyl. Furthermore, handling of pyrophoric tert-butyllithium, even in a highly dilute solution is extremely expensive and difficult on an industrial scale for safety reasons.
Various metal salts have been tested for their catalytic activity in increasing the yield in reactions of the cheaper metal alkyl, tertbutylmagnesium Grignard, with halogenosilanes to give the corresponding tertbutylsilanes. According to EP-A-405 560, satisfactory results were achieved with cyanide- or thiocyanate-containing catalysts such as silver cyanide, mercury(II) cyanide, copper(I) cyanide, sodium thiocyanate or copper(I) thiocyanate, but these involve major safety, environmental and disposal problems. In addition, the silanes prepared via cyanides and thiocyanates usually have an unpleasant smell, which can reduce the quality or capacity for use of the corresponding silane.
When more acceptable halogen-transition metal catalysts were used, to date, only the reactions of Grignard compounds with hydrogen-containing chlorosilanes, described in EP-A-542 250, and the reactions of Grignard compounds with chlorosilanes containing at least three chlorine atoms, described in EP-A-656 363, gave acceptable yields.